How much ventilation for diesel?

[Jdege]

Originally Posted by BillKny

For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

(2.75 x HP) - 90 = cu. ft/min

It's a 27hp engine, so the minimum passive would be 8.2in^2. which is far less than the 25in^2 that the two 4in holes provide.

And as for forced air, the formula yields -15.75ft^3/min, which suggests that it doesn't need forced air.

Plus, given where the engine sits, it can pull air through all of the cockpit lockers, none of the lids of which are air tight.

All of which suggest to me that restricting the flow of the two existing holes a bit, by adding a baffle to make it harder for water to get in, would not cause problems.

Thanks.

[Jim Cate]

Well, we've had the theory now ( and I agree with those who say that a diesel will suck in about half its displacement per revolution, averaged out.

But the nitty: make up a simple manometer with some clear plastic tubing. One end goes to the engine room/box, the other to the cabin air. Run engine at various speeds and see how much pressure drop there is in the box with various vent schemes.Now you have data which you can send to the engine mfg for evaluation.

But I strongly suspect that you will find that it gets plenty of air with only one 4 inch vent plus all the other sources that exist in most boats.

Jim

[Compass790]

Originally Posted by Jim Cate

Well, we've had the theory now ( and I agree with those who say that a diesel will suck in about half its displacement per revolution, averaged out.

But the nitty: make up a simple manometer with some clear plastic tubing. One end goes to the engine room/box, the other to the cabin air. Run engine at various speeds and see how much pressure drop there is in the box with various vent schemes.Now you have data which you can send to the engine mfg for evaluation.

But I strongly suspect that you will find that it gets plenty of air with only one 4 inch vent plus all the other sources that exist in most boats.

Jim

There's a major source of hot air in our boat.....
Funnily enough it's related to beer, not sure of the exact formulae but its a logarithmic progression
Whoops I mite be heading for a thrashing again.

[GordMay]

Originally Posted by BillKny

... Here is the actual answer: (Ref, David Gerr, "The Nature Of Boats")

For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

With that said, more is ALWAYS better.

Calculating the amount of air the engine actually sucks in is only part of the equation. You have to cool the room, and parts like the alternator. That, and the cooler the air ingested by the engine, the more efficient it is.

Far better to have blowers actively move the air, and the amount required looks like this:

(2.75 x HP) - 90 = cu. ft/min

So a 100 hp engine needs:

(2.75 x 100) - 90 = 185 cu ft / min

Again, MORE IS BETTER.

See also, some earlier CF discussions:


Engine Ventilation: Skip it?
➥ https://www.cruisersforum.com/forums...-it-37561.html

Engine room ventilation
➥ https://www.cruisersforum.com/forums...n-85829-2.html

Engine Room Blower
➥ https://www.cruisersforum.com/forums...wer-42656.html

Engine ventilation
➥ https://www.cruisersforum.com/forums...tion-7269.html
Engine Bay extractor fan
➥ https://www.cruisersforum.com/forums...an-158145.html

Caterpillar has an excellent guide, which seems to go missing (links don’t work), requiring a search, each time.
“Engine Room Ventilation” [Caterpillar publication # LEBW4971-03]
➥ https://www.scribd.com/document/1061...on-LEBW4971-03

[Squanderbucks]

GordyMae. There is more need for an engine room than just what the engine consumes. Anyway when engineers do all the fancy calculations they often end up after their all done adding in a + fudge factor to be safe. Safe bet the route or re-route what you have as long as yo go no smaller. When I added a larger genset and repowered from 427 cui gas to 5.9L diesel I added additional passive and active air intakes as well as fan exhausts. I installed a temperature probe and watched what the engine room temp was with various configurations and went from 145 deg F down to 115-118F. Engine room designers recommend max engine room temps below 120F for engine and accessory life expectancy. Always avoid the minimum of most things and see what the maximum is you can reasonably provide - all things considered.

[walbee]

The attach Cat Guide I found helpful, although a bit math intensive, it does reveal variables I had not considered.

Originally Posted by Jdege

I have a boat with a Yanmar 3GM30, that had it's engine compartment vents removed for painting.

I could simply replace what was there, but I'm not really thrilled with the location or design.

But before I do anything different, where could I find out how mugggch air intake a 3GM30 requires?

And how large a vent area would be required to provide this?

[jdaltonpe]

In my experience more air is better all the way around to keep temp down and alternators happy, especially if it is via natural aspiration like dorad vents. One advantage that I don’t see mentioned is air exchange at anchor which seems to help keep corrosion at bay in the engine room.

[GordMay]

Originally Posted by Squanderbucks

GordyMaeGordMay. There is more need for an engine room than just what the engine consumes ...
... Always avoid the minimum of most things and see what the maximum is you can reasonably provide - all things considered.

Indeed.

[Hillbilly]

Originally Posted by Bill Seal

A diesel engine inhales about 1/2 its displacement per revolution, a gas engine running 7 inches of vacuum inhales only half as many molecules per revolution.

Bill Seal finally gets to the heart of it. It's not compression, it's air/fuel ratio.
In order to get reliable spark ignition and burn properly with a mixed incoming charge from the carburetor or equivalent, a gas engine must maintain an A/F ratio within narrow limits. This is done by "throttling" the air inflow to match the load on the engine. The resulting manifold vacuum requires the engine to consume extra energy in order to act like a vacuum pump. This probably is the main source of diesel efficiency advantage.

The diesel runs with hardly any inflow restriction so its air consumption is greater and depends almost entirely on rpm. A/F ratio doesn't matter because fuel is introduced directly into the combustion chamber as a mist and self ignites. The fact that there normally is an excess of air promotes more complete combustion. Notice the over-sized exhaust pipes on diesel pickups.

[OS2Dude]

Our 1985 Catalina 30 has similar vents. To my understanding, it is because originally Catalina used Automic4 gasoline engines which required venting with blowers to remove any vapors from around the fuel tank (Stbd vent) and the engine (Port vent). Our boat has a Universal M25 diesel (which does not require venting for vapors) but still has vents. I use the engine vent & blower to pull heat and smell from the cabin. (Engine is under the settee and galley cabinet in main saloon.)

If your boat has vestigial vents like ours, you could likely close them off. It may be hard to get a good gelcoat color match and finish though.

[BillKny]

Originally Posted by Hillbilly

The resulting manifold vacuum requires the engine to consume extra energy in order to act like a vacuum pump. This probably is the main source of diesel efficiency advantage.

Actually... not.

The biggest source of diesel "efficiency" is just the simple fact that we buy fuel and measure its usage by volume, and diesel fuel is significantly heavier than gasoline. Diesel contains more energy per gallon than gasoline.

If we measured fuel usage by weight of fuel consumed (miles/pound or kilometer/kilogram) you'd see that diesel engines are actually very similar in efficiency to gasoline engines in converting the energy contained in the fuel to actual work.

Numbers: Gasoline: 0.75 grams/liter Diesel: 0.85 grams/liter

So making the very accurate assumption that gasoline and diesel have the same energy density available per gram of fuel burned, with nothing else happening at all you get 13% more power out a liter of diesel fuel as you do from gasoline.

[adjo]

It's good practice to allow for airflow around the engine to assist in cooling and to cool the engine compartment itself: especially in the tropics and warm waters. So inlet and outlet should be significantly bigger than that which is required for combustion air alone.
All the air used in combustion exits via the exhaust anyway (hopefully!).

As mentioned above, many boats suck air from the bilge which can help keep the air in this area fresh. A well designed boat, however, should have a sealed engine room/compartment that can be effectively acoustically and thermally insulated (and closed off in case of fire). Without a generous air inlet/outlet the entire compartment gradually heats to the temperature of the motor.

[NorthCoastJoe]

Get a probe thermometer and run it hard with vents open then try it with vents restricted. I would keep the air temp below 120 F. They put intercoolers on turbo diesel for a reason. Cool air is denser more oxygen.

I think the issue is more about cooling air than combustion air. If you close it up too much expect problems. On my boats, I have a lot of ventilation into and out of the engine compartment.

[Jdege]

Originally Posted by OS2Dude

If your boat has vestigial vents like ours, you could likely close them off. It may be hard to get a good gelcoat color match and finish though.

The original Herreshoff Meadow Larks had gas engines. The Vaitses Meadow Larks all seem to have been one-offs, with very little standardization. Curlew may well have had gas engines to start.

What I'm thinking I might do, one I have her in the water, is to seal the vents, run the diesel with a cockpit lockers open, then close the locker and see what happens.

It may be that there is no real need for vents.

[petermck]

I recently went down this route during a rebuild and was surprised by the result.
The rules of thumb I was told is app. 50"sq per 100HP on marine diesel engines and 1”sq per HP for convection cooling.

After doing some number crunching I arrived at the following-
Intake airflow for combustion of both engines is 585 CFM
Free air volume in the engine room is 3.3m and should be changed twice every minute = 110 l/s
30% volume to be added for use in tropics (Indonesia, Malaysia, Thailand) = 143 l/s or 302 CFM
The total air intake required is app. 900 CFM through 2 moisture eliminators.

9” fan will pull 600 – 700 CFM into the engine room and 1 off 320 CFM extraction blower is needed but maintaining slight negative pressure within the engine room would require 2 blowers.
Maximum airflow velocity of 3 m/s is also a consideration when sizing intakes.

The original intakes were only providing 30% of what the engines required. It appears that the vacuum created at WOT sucked air from the engine crankcase and helps understand why the air intakes and turbos had a film of oil inside and aftercoolers were clogged with shellac.

I ended up getting some moisture eliminator intakes (single-stage vane separators) fabricated by Camfil in NZ, DeltaT and a few others produce similar products.

I hope that this helps.